The previous GData API and Javascript API will still be available, but ongoing development work will be focused on the new Google Books API so we encourage you to make the transition to take advantage of new features including:

We’re excited about some of the new capabilities this API gives our developer partners. Using OAuth 2.0, a developer can allow users to retrieve their stored information, including bookshelves in their library, ratings and reviews. With the new JSON structure, accessing this information can be done with ease. Whether you want to use the API on a web page, in a Chrome extension, or on a device, the new Books API gives you a way to access the world’s largest selection of ebooks.